1. Field of the Invention
The invention relates generally to switches used in optical communications networks. In particular, it relates to optical routers and optical buffers used therein.
2. Background Art
Two important recent advances in communication networks include large, high-capacity networks based on packet switching and optical fiber transmission media, in particular silica fiber. More recent further advances include asynchronous packet switching, such as the TCP/IP protocol of Internet networks, and wavelength-division multiplexing (WDM) of multiple optical signals on a single fiber. Combining these advances has presented difficulties.
Heretofore, the switching of asynchronous packets has been typically performed by electronic routers, which require electro-optical conversion of optical signals before and after switching. Electronic routers in WDM networks are difficult to scale to a large number of channels and present other problems of carrying different types of traffic on a large network.
An optical router is greatly desired to perform the packet switching without converting the data payload of the packet to optical form. Yoo has described such an optical router in U.S. patent application Ser. No. 09/654,384, filed Sep. 1, 2000, now issued as U.S. Pat. No. 6,519,062, incorporated herein by reference in its entirety. Yoo has also described the integration of such an optical router on a single substrate in U.S. patent application Ser. No. 10/081,396, filed Feb. 22, 2002, now issued as U.S. Pat. No. 6,768,827, incorporated herein by reference in its entirety.
A principal problem with optical routers is the difficulty of implementing optical memory, particularly random access optical memory. Some sort of memory is desired when two packets arrive on different source fibers and both require transmission to a same destination fiber, that is, memory to resolve contention. The multiple channels of a WDM fiber can alleviate some contention, but at the expense of excess capacity. TCP/IP allows some packet loss if the contention cannot be resolved, but the loss rate needs to be minimized. A feedback fiber of substantial length can be installed from the output side to the input side of the router to provide a fixed delay storage, but the feedback route complicates the router control and may contribute to further delay. Proposals have been made for parallel banks of selected fibers of different lengths. However, such an arrangement splits the optical power and requires bulky fibers.
Several approaches to packet buffering are described by Zhong et al. in “A new wavelength-routed photonic packet buffer combining traveling delay lines with delay-line loops,” Journal of Lightwave Technology, vol. 19, no. 8, August 2001, pp. 1085-1092 and by Hunter et al. in “Buffering in optical packet switches,” ibid., vol. 16, no. 12, December 1998, pp. 2081-2094.